Radio propagation is generally governed by three basic propagation mechanisms: reflection, diffraction, and scattering. Multipath fading and shadowing are two types of large scale fading occurrences for signals. The reflection of various types of signals causes fluctuation in the phase and amplitude of the received signal—known as multipath fading. Multipath fading occurs when communicated signals travel through multiple paths from a transmitter to a receiver and consequently arrive with different time delays, phases, and power levels. Shadowing refers to the distortion as a result of diffraction and scattering loss. The severity of fading occurrences can depend on the local topography of the area surrounding the receiver(s).
Current attempts to mitigate the effects of fading in wireless communications have not been successful. Mechanisms to minimize multipath fading include implementing antenna diversity, equalizers, and RAKE receivers in base stations/receivers. Methods to minimize shadowing include implementing macroscopic diversity and fade margin. Macroscopic diversity is effective, but requires wide special separation of antenna branches—unfeasible for mobile devices. Typically, several decibels of link budget are reserved as fade margin to accommodate signal variation. However, a high fade margin requires dense cell deployment, resulting in high output power and more expense on the cellular operator.
The fading issues are multiplied for indoor coverage. Depending on the building type, size, materials, age, and configuration, indoor signals must be strong enough to cover the approximately 20 dB or more penetration loss when traveling through walls and windows. Furthermore, high speed data applications that use complex modulation and coding schemes require an even higher signal to noise ratio. Because of the inability to accurately locate an indoor user, implementations of E-911 location technology, such as Enhanced Time Difference of Arrival (E-TDOA) and Assisted GPS (A-GPS), are ineffective. Additionally, base stations that are deployed outdoors can require high output power and/or dense placement—either of which create interference for outdoor users. Base stations deployed indoors are expensive, unless significant in-building traffic can be identified.